1. Field of the Invention
This invention relates in general to a bearing arrangement for an electric motor. More particularly, the invention provides a bearing of the general type having an inner ring, an outer ring, a retainer and a plurality of rolling elements arranged in a spaced relationship between the inner and outer rings and secured by the retainer so as to be rollable.
2. Description of Related Art
A known roller bearing is shown in FIGS. 11 and 12 (Prior Art). The roller bearing shown is of the type intended to supporting a rotor of an electric motor, typically of the type used as a scanner motor employed in a laser printer.
FIG. 11 (Prior Art) is a perspective view of a retainer of the known bearing, and FIG. 12 (Prior Art) is an enlarged front view of the known bearing.
The known bearing comprises an inner ring (not shown), an outer ring (not shown), an annular retainer 2 having a plurality of receiving portions 1, and a plurality of rolling elements, such as metal balls, 3 disposed in the receiving portions, respectively. Retainer 2 is shaped so as to define a plurality of grease reserve portions 4 between adjacent pairs of receiving portions 1 along an upper side of the retainer, as viewed in FIG. 11.
As shown in FIG. 12, a lubricating grease 5 is injected into each of the grease reserve portions 4. The lubricating grease 5 is composed generally of a base oil, such as mineral oil, and a thickening agent having relatively high viscosity, such as metallic soap. The scanner motor is usually a vertical shaft type motor in which its rotational shaft extends vertically and is operated in a high speed range of 8,000 to 15,000 rpm.
In known scanner motors, a speed variation sometimes occurs momentarily during driving of the motor. FIG. 13 is a graph showing speed variations of a known motor utilizing the bearing shown in FIG. 11. This momentary, large scale speed variation is referred to conventionally as `jitter`. Jitter causes fluctuation in the image printed by a laser beam printer and is therefore highly undesirable.
The present inventor has investigated the cause for jitter in scanner motors of laser beam printers. The investigation revealed that the grease is subjected to a centrifugal force due to rotation of the retainer. The force causes the grease to scatter out of the grease reserve portions and to deposit on the outer peripheral side of the retainer. Thereafter, the scattered grease flows excessively onto the rolling surfaces of the balls. This problem tends to occur more frequently as the temperature of the grease rises. The momentary variation in motor speed occurs when the balls contact the grease or, more specifically, a thickening agent of the grease.